The exhaust gas of an internal combustion engine contains harmful substances such as nitrogen oxides (NOx) and the like. It is known that an exhaust system of an internal combustion engine is provided with a NOx catalyst to remove NOx from the exhaust gas in order to reduce emissions of these harmful substances. With this technology, for example, when a NOx storage-reduction catalyst is provided, the purification ability drops if the amount of stored NOx increases, so a reducing agent is supplied to the NOx storage-reduction catalyst, and the NOx stored in the catalyst is reduced and discharged. (Hereinafter, this process is called “NOx reduction processing”.) Sulfur oxides (SOx) in the exhaust gas are also stored in the NOx catalyst, which results in SOx poisoning that reduces the purification ability of the catalyst. To eliminate the SOx poisoning, sometimes the bed temperature of the NOx catalyst is increased and a reducing agent is added. (Hereinafter, this process is called “SOx regeneration processing”.)
It is also known that when a reducing agent is supplied to an exhaust gas purification device, such as a NOx catalyst or the like, and the purification ability is regenerated, as described above, it is desirable to reduce the flow volume of the exhaust gas that is introduced into the exhaust gas purification device, so as to ensure sufficient time for diffusion and reaction of the supplied reducing agent within the exhaust gas purification device.
For this purpose, technologies have been proposed, as disclosed in Japanese Patent Application Publication No. JP-A-2003-106142 and Japanese Patent Application Publication No. JP-A-2003-74328, whereby a plurality of branch passages are provided in an exhaust gas purification system, and an exhaust gas purification device is provided in each branch passage. (Hereinafter, the expression “exhaust gas purification system” is used to refer to one or more exhaust gas purification devices and an associated control system.) With these technologies, the flow volume of the exhaust gas that is introduced into any one of the exhaust gas purification devices is decreased to a prescribed volume by means of a valve that varies the cross-sectional area of a flow passage, and fuel is supplied as a reducing agent to an exhaust gas purification device for which the flow volume of introduced exhaust gas has been reduced. This allows the supplied fuel to be used efficiently to regenerate the purification ability of the exhaust gas purification device. It also limits the effect on the operating performance of the internal combustion engine. A related technology is proposed in, for example, Japanese Patent Application Publication No. JP-A-2004-52603. With this technology, when NOx reduction processing is carried out for the NOx catalyst, a switching valve is changed from a forward flow setting to a reverse flow setting or vice versa. By this method, the volume of exhaust gas within the NOx catalyst is reduced by changing a valve setting. The reducing agent is supplied after a prescribed amount of time has elapsed from the time the valve switching signal was generated.
However, with the technology described above, when the flow volume of the exhaust gas in the exhaust gas purification device was reduced at a constant speed, for example, there was a risk that the reducing agent would not arrive at the exhaust gas purification device with the timing at which the reducing agent should be added. There was also a risk that the reducing agent would pass through the exhaust gas purification device and escape. As a result, it was sometimes difficult to supply the reducing agent to the entire exhaust gas purification device reliably and to regenerate the purification ability efficiently.
The present invention provides a technology that more reliably or more efficiently regenerates the purification ability of an exhaust gas purification device in an exhaust gas purification system that combines a plurality of branch passages that branch off from an exhaust gas passage and an exhaust gas purification device that is provided in each branch passage.